Defense Grid: The Awakening has a simple story that is often used when a game designer wants a generic enemy in their game and having Nazis don't make any sense. That's right, Defense Grid features an alien invasion. On a far away planet you wake up to find everything in chaos. Cities are abandoned, their defense networks completely disabled, and its up to you to save the power cores which will save the human colonists the inconvenience of have to cook with natural gas. There could be much more explained here it isn't. Instead of attacking the humans directly why do the aliens try to steal power cores?